A clear trend in satellite communications is towards the use of multiple spot beam coverage in order to provide high gain (to reduce power and operate with small terminals) and to support spatial frequency reuse (to increase throughput within bandwidth constrained systems). It is known accurately to define such narrow spot beams, with a multi-element antenna system, by digital beam-forming techniques involving assigning complex digital weights to the elements for each communication frequency channel for controlling spot beam parameters: see A. M. Bishop et al “The INMARSAT 4 Digital Processor and Next Generation Developments”, 23rd AIAA ICSSC, Rome, Italy, September 2005. An ongoing problem is that, although there are continual improvements in enabling digital processor technologies, there are continually increasing demands in terms of beam-forming and associated processing associated with the need to provide larger numbers of narrower spot beams operating over broader bandwidths.
An important class of antenna which may provide such coverage is the phased array antenna, either in its direct radiating array (DRA) or imaged radiating array (IRA) form. In the case of a DRA, the antenna may be comprised of a two-dimensional matrix of antenna elements, typically but not necessarily identical, each antenna element having a respective element signal (operating in either transmit or receive mode). In an IRA, the aperture diameter of a primary DRA is magnified by means of antenna “optics” (e.g. parabolic reflectors). For the purposes of the present specification, the term “phased array antenna” includes a DRA, IRA and other arrays, having a plurality of antenna elements, each of which provides an antenna element signal having a phase relationship and an amplitude relationship to the other element signals. Phased array antennas offer high performance in terms of flexibility and reconfigurability resulting from control of element amplitude and phase weights within a beam-forming network. But it is also well known that phased arrays are complex, massive and expensive. Compared with alternative antenna types, for instance array-fed reflectors (AFR), they typically require many more radiating elements and thus much greater beam-forming complexity and cost.
A beam-forming network provides reconfigurable amplitude and phase control (equivalent to complex weights in the digital domain) for each antenna element (and potentially on an individual frequency channel basis) such that beam-forming complexity scales with the number of elements. Even the next generation of on board digital processors cannot support the complexity needed to provide fully flexible beam-forming for a phased array designed to generate (for instance) beams of diameter 0.5° covering Europe at a frequency of 20 GHz with a bandwidth of 500 MHz, which is a current commercial requirement.
It is known to simplify phased array antenna construction in various ways. In particular, it is known to partition phased array antennas into sub-arrays, and this may offer simplifications in construction and signal processing.
Overlapping sub-arrays are described in “Design Considerations and Results for an Overlapped Sub-array Radar Antenna”, Jeffrey S. Herd et al, 2005 IEEE Aerospace Conference, pp. 1087-1092.